In a mobile communication system, especially a cellular system, a wide communicable service area is secured by arranging a cover area of the base station whose cell radius is different from each other so that the cover areas overlap each other. However, the signals of the base stations interfere with each other when the cover areas of. the base stations overlap each other and whereby there is a risk of decrease in line capacity. A probability of existence of the base station having no mobile station in the cover area of the cell rises with the increase of the number of the base stations. Although such base station is not used, it continues the operation and consumes an electric power needlessly.
Therefore, a technology for avoiding interference and reducing the consumed electric power in a mobile communication system is proposed. For example, patent document 1 (Japanese Patent Application Laid-Open No. 2003-37555) discloses a technology that decreases the number of base stations that operates at light traffic and interference to the neighborhood base station by monitoring a downlink signal transmitted from another base station in own base station and stopping or starting transmission from own base station in consideration of traffic condition of another base station and a received power.
As the related art, in patent document 2 (Japanese Patent Application Laid-Open No. 2001-78246), there is shown that a primary base station selects a suitable handover candidate by using a measured value of signal quality of an adjacent base station. A technology in which when another wireless base station that has better communication quality than the wireless base station with which communication is currently performed exists, the channel during communication is switched to the channel of another wireless base station is disclosed in patent document 3 (Japanese Patent Application Laid-Open No. 1991-117121). A technology in which when a total uplink interference level or a total downlink transmission power of an adjacent wireless base station is equal to or greater than a specified value, a handover of a mobile terminal under the control of the base station to the adjacent cell is suppressed is described in patent document 4 (Japanese Patent Application Laid-Open No. 2007-295318).
As the related art, a transmission power control technology in a cognitive wireless system is disclosed in non-patent document 1 (Mochizuki, Takyu, Umeda, Fujii, Nakagawa, “Fundamental Study of Cognitive Radio System Using Distance between Transmitter and Receiver Specified by Detection of Modulation Scheme in Adaptive Modulation Technique” Proceedings of The 2008 IEICE Communications Society Conference, B-17-14, 2008). In this technology, first, the cognitive wireless system estimates a modulation scheme by using a downlink signal transmitted from the base station in another wireless communication system using an adaptive modulation/demodulation and specifies a distance between the base station and the mobile station from its modulation scheme. The cognitive wireless system performs a transmission power control so that an amount of interference to the mobile station is equal to or less than a fixed value at a position of the mobile station at which the amount of interference to the mobile station is maximum (a position at which the cognitive wireless system and the mobile station are most closely located) by considering the above-mentioned distance and shares the same frequency band with another wireless communication system.